1. Field of the Invention
The present invention relates generally to the field of spray guns for dispensing materials, and more particularly, to a spray gun for dispensing a plurality of reactive components and a solvent system for cleaning the gun of combined reactants. Still more specifically, the invention relates to a gun which may be efficiently solvent cleaned and yet which may readily be altered to provide air cleaning, if desired.
2. Description of the Prior Art
The application of plural component systems using a spray gun is well known. While a variety of such systems are known in the prior art, the present invention and the prior art relevent thereto can best be illustrated by reference to the application of polyurethane foams using such spray guns.
Polyurethane foams are typically prepared by reacting a first component comprising a polyol and a second component comprising a polyisocyanate. Other materials which are commonly employed in urethane foams, such as blowing agents, cell formers, etc. may be included in one or the other of the two major components. When mixed, the components react to form a foam useful for insulation, packaging, coatings and the like. The reaction takes place very quickly, in most cases within 10-20 seconds. The resulting foam may be rigid or flexible.
One major difficulty experienced in the prior art has been the cleaning of the guns used for foam application, due mainly to the fact that such guns typically include some type of mixing chamber for the plural components. Some guns attempt to overcome the problem by providing for the convergence of separate streams of components at a location in front of the tip of the gun, but these devices have not received widespread commercial acceptance due to the fact that intimate mixing is difficult to achieve. Other types of guns employ the more typical approaches of providing for compressed air or solvent flushing of the mixing chamber. Several examples will now be described so that the advantages of the present invention can be more fully appreciated.
One such system is described in commonly assigned U.S. Pat. No. 4,325,513, issued Apr. 20, 1982 to Robert L. Smith and Gary L. Smith and entitled "Gun for Dispensing a Plural Component System." In this patent, an air flush system is provided in connection with a gun having a forward static mixing chamber and inlet ports leading thereto for the supply of first and second components. Compressed air is connected to the gun and an air passage leads to the mixing chamber and is selectively coupled thereto by a valve member. The inlet ports are selectively closed by other valve members. A trigger mechanism on the gun is coupled to the respective valve members in such a way that when it is placed in a first position, compressed air is admitted to the mixing chamber to purge same. When the trigger is placed in a second position, the air purge is terminated and the components of the plural component system are discharged from the respective component feed ports, into the mixing chamber and out of the discharge outlet. In the preferred embodiment of this patent, the second trigger position is achieved only after the trigger has been pulled through the first position, and the gun is shut down only by going back through the first position, thus insuring that an air blast is always employed to purge the mixing chamber.
The gun described in the Smith et al. patent has become commercially accepted and has satisfied many of the problems which existed in the art prior to its introduction. However, the gun of that patent does have certain drawbacks. In some installations where this type of gun would be employed, compressed air may not be available or the noise resulting from a compressed air purge may not be acceptable. Moreover, for some spray applications, the mixed components may be so difficult to remove by an air purge that frequent downtime is required for gun cleaning. Furthermore, it may be desirable in some installations to have the alternate capabilities for air or solvent cleaning, a capability not found in prior art devices known to the present inventor.
Other prior art guns employ solvent cleaning by attaching solvent lines to the gun and various additional valve arrangements to permit solvent to flow through the mixing chamber after spraying of the plural components. Some of such guns also provide for solvent lubrication of moving parts within the gun to keep them from freezing following contact with mixed components.
Still other types of guns employ a clean out rod for the mixing chamber, an example of which is Gusmer et al. U.S. Pat. No. 2,890,836, issued June 16, 1959 for "Apparatus for Applying a Mixture of a Plurality of Liquids." In the gun described in this patent, the plural components are admitted tangentially to a cylindrical bore mixing chamber near the gun tip, and a cylindrical rod is controlled by a tigger mechanism to move back and forth through the mixing chamber. The rod serves the dual purposes of opening and closing the component inlets to the chamber as it is drawn away from the tip and of cleaning out the mixing chamber as the trigger is released and the rod moves to the front of the bore. In this patent, the rod has a conical tip with a sharp point so that the tip extends outside the mixing chamber to expel mixed components therefrom.
In subsequently issued U.S. Pat. No. 3,263,928, issued Aug. 2, 1966 for "Apparatus for Ejecting a Mixture of Liquids", Gusmer describes a similar gun in which an elastically deformable material forms a portion of the chamber wall. The bore formed by such portion has a diameter which is slightly less than the diameter of the cleaning plunger. The plunger expands the portion when in its forward position to insure cleaning contact. This patent further discloses an air cap at the front of the gun so that compressed air can be blown across the tip to remove mixed components therefrom. The plunger in this patent is controlled by compressed air mechanisms and a trigger valve.
More recent patents also disclose a rod or plunger cleaning process for such spray guns. For example, Hagfors in his U.S. Pat. No. 3,786,990, issued Jan. 22, 1974 for "Plural Component Gun" describes a plunger in which the front of the plunger is configured in a particular way to create different spray patterns. In this patent, another improvement is suggested, i.e. the addition of a lubricant for the rod supplied rearwardly of the plural component inlets. The lubricant may be gravity fed or may be fed under pressure from a reservoir in the gun. Hagfors also suggests the use of fluid jets near the tip to clean the front of the rod before the plural components have time to react on the rod.
A still further example of a plunger-type gun is shown in Sperry's U.S. Pat. No. 3,945,569, issued Mar. 23, 1976 for "Foam Dispensing Apparatus." A reciprocally mounted, air pressure controlled valve rod has a circular cross section at its forward end to pass through and clean the mixing chamber and to open and close the inlets for the plural foam forming components. However, the rod has a flattened portion between its ends and a reservoir of cleaning solvent is provided in the body of the gun through which the rod passes during rearward movement. The solvent cleanses the rod and allows it to move freely through the mixing chamber.
While these latter patents are embodied in various commercial spray gun devices, they suffer from certain problems, such as sticking of the valve rod. Also, air cannot be used for cleaning if that is desired in a particular spraying application. A spray gun which may be easily cleaned by solvent would represent a significant advance in the art, especially if the gun could readily be adapted for either solvent or air cleaning.